The invention concerns a device for suppressing flow-driven resonances in a diffuser.
It is generally well known that, for example in valve construction, flow-driven acoustic resonances appear in the connector to the restrictor when installing a diffuser. For the suppression of these very disturbing effects, different variations of restrictor geometry were tested. In this way, it was, for example, determined that bulb-shaped valve disks featured an unfavourable behavior with respect to the above-mentioned acoustic effects.
The control of the undesired vibration effects by measures taken at the restrictor features various disadvantages. For one thing, in many cases, the instability can thereby not be suppressed. For another thing, the optimum configurations desired for reasons of stability are not possible for reasons of flow losses or for reasons of mechanical stability.
It is known that the diffuser essentially encourages the appearance of a vibration and further that sonic waves run from the restrictor to the diffuser outlet and are there reflected. At this point flow energy can be removed whereby the resonances are induced. Accordingly, it is a primary object of the present invention to prevent the appearance of flow-driven acoustic resonances and at the same time to avoid additional flow losses.
According to the invention, the primary object is achieved in that a cross section modification is provided at the diffuser outlet in order to modify the acoustic impedance.
The resonance cycle in the diffuser is interrupted in the critical frequency range by an appropriate impedance modification of this type.
Since diffusers in the various flow turbines and devices customarily terminate either in an annular chamber or pass over into a continuous pipeline, the cross section modification can be an expansion, a contraction or a deflection at the diffuser outlet which is attached either symmetrically or asymmetrically at the outer circumference or in the case of ring or hub diffusers at the outer and/or at the inner circumference.
Since the diffusers are designed owing to flow-technical reasons to be as steady as possible, almost uniform surface expansion takes place along the diffuser, the modification of acoustic impedance at the diffuser outlet can be achieved in a specified frequency range of the sonic wave such that the cross section modification at the diffuser outlet essentially occurs rather quickly, i.e., on a section which is comparable with the wavelength of the sonic wave to be suppressed.
The advantage of the invention is more particularly to be seen in that the measure to be carried out can be controlled very well. Thus, it is possible for the minimum impedance modification required for a given diffuser inlet to be accurately specified. It is basically possible thereby for any vibration to be modified for whose appearance the diffuser is authoritatively responsible. A further advantage can be seen in the fact that no intervention has to be made in the more critical flow restrictor part in front of the diffuser inlet.